Direction finding system



March 10, 1942. F E TERMAN 2,276,119

DIRECTION FINDING SYSTEM Filed Feb` 23, 1940 3 Sheets-Sheet l FIGA@ i naw,

TFH/HAN March 10, 1942. F. E. TERMAN DIRECTION FINDING SYSTEM Filed Feb. 25, 1940 3 Sheets-Sheet 2 FIGA.

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March l0, 1942. F. E. TERMAN DIRECTION FINDING SYSTEM Filed Feb. 23, 1940 5 Sheets-Sheet 3 FIC-5.8.

lllll IUI! uA/vc/A/G' Cowan/SER n/ u! @LIYCY/V6' wiwi/ufff? INVEN TOR. FREDERIC/f E P/ff/V A ORNEY.

Patented Mar. 10, 1942 ES PTENT OFFICE DKRECTION FINDING SYSTEM Delaware Application February 23, 1940, Serial No. 320,338

(Cl. Z50-11) 2 Claims.

This invention relates to radio direction iinding systems and more particularly to improvements in antennas associated with said systems.

An object of my invention is to compensate for night eiect errors in radio direction iinding by arranging an auxiliary antenna to operate in conjunction with ordinary rotatable loops.

By my invention I neutralize the undesired voltages induced in a loop antenna by horizontally polarized waves by means of an equal and opposite voltage induced in an auxiliary antenna mounted on the loop. This auxiliary antenna is mounted above or near the loop and is rm- 1y attached to the loop so that the loop and the auxiliary antenna rotate together.

My invention will be more clearly understood by the use of the attached drawings wherein:

Figs. la and 1b illustrate the voltages induced in a rotatable loop;

Figs. 2a and 2b illustrate the voltages induced in a rotatable loop by a wave having both vertical and horizontal components;

Figs. 3 and 4 are space diagrams oi a loop and the earth used in describing my invention;

Figs. 5 through 7 are vector diagrams used in describing my invention;

Fig. 8 illustrates an embodiment of my invention.

. When a vertical loop is so rotated that its plane is at right angles to the direction of arrival of the wave as in Figs. la and 1b, then a vertically polarized wave will induce voltages E1 and Ez of equal magnitude and identical phase in the two vertical sides of the loop. The two voltages therefore balance each others effects and give zero loop output. With other loop positions the voltages induced in the two vertical members are still of equal magnitude, but now have a phase dilerence so that there is a resultant voltage acting around the loop that develops output. Thus by rotating the loop until zero output is obtained one can obtain the bearing angle of the wave.

Assume now that in addition to the vertically polarized wave just discussed there is also present a horizontally polarized downcoming component, as illustrated in Figs. 2a and 2b. This component induces voltages e1 and ez in the horizontal members of the loop even when the loop is in the null position for the main wave. In fact, these voltages are maximum when the loop is in the normal null position. Now the voltages e1 and e2 are not equal in magnitude and also have different phases, so that they never cancel each other. As a consequence, the horizontally polarized downcoming component of the wave causes the loop circuit to develop output voltage even when the loop is in the normal null position. The operator, in attempting to obtain a null under such conditions will rotate the loop away from the position indicating the true bearing until there is suiiicient output developed by the main Wave to annul the spurious voltage ei-ez acting around the loop circuit as a result of the downcoming component. The result is hence a null indication in an incorrect position of the loop. In some cases, the phase ei-ez is such that there is no loop position for which cancellation of this spurious voltage is obtained. Under such conditions a blurred and usually erroneous minimum is observed.

The downcoming wave commonly arises from reection produced by the ionosphere. Since at the frequencies most widely used indirection nding work the ionosphere reflections are small or entirely absent during the day, and relatively strong at night, the error from downcoming waves is commonly referred to as night eiect. It will, however, be present even in the daytime at certain radio frequencies, and will also be present when signals are received from an airplane that is overhead.

In accordance with my invention I neutralize the undesired voltage e1-e2 induced in the loop by horizontally polarized downcoming waves, by means of an equal and opposite voltage induced in an auxiliary antenna mounted on the loop. I mount this auxiliary horizontal antenna in the plane of the loop preferably above the loop as shown in Fig. 3, and rmly attach it to the loop so that the loop and auxiliary antenna rotate together.

The following analysis shows how the voltage induced in the auxiliary antenna has the characteristics required to accomplish the desired cancellation under all conditions. Consider rst the voltage induced on a horizontal wire by a horizontally polarized downcoming wave as illustrated in Fig. 4. Let

0=ang1e of incidence measured from normal En=vector field strength of horizontally polarized component of the incident Wave E'H=reected Vector at ground EH=reected at wire h=height of wire above ground.

It will be assumed that the ground has a reflection coeflicient of unity and that the reilection takes place with a reversal of phase phase shift) This ideal of a perfectly reflecting earth is almost completely realized over water, over good earth at thel frequencies commonly used in direction finding work, or over craft having conducting bodies. The antenna is acted upon by two waves, a direct wave EH and a wave E"H reilected from the earth. The magnitudes of these two waves are the same for perfect reflection but the phase at the antenna diiers because E"H has had to travel an extra distance 2h cos 0, and has also had its phase reversed by the reflection. As a result, the two waves acting on the antenna have a phase diierence of -i-1, radians, where 62212-7? cos f6 (l) The total voltage ET induced perr unit length in the horizontal wire is then the vector sum .of Err and EH, taking into account this phase difference existing between these voltages.

From vthe Vector diagramy of Fig, it is apparent that ET has the magnitude lETl=41rEH cos 0 (2a) As seen from Fig. 5, the voltage vET leads the voltage EH by the angle (1r-16H2 where is the angle given byrEquation l. One can `now calculate the voltages e1, e2, and e3 induced in the horizontal'members in Fig. 3 by Ythe horizontally polarized and downcoming wave. Substitution in Equation 2a immediatelygives the results The phases of these voltages expressed with ref spect to the phase of EH are given in Fig. 6, where the subscripts 1, 2, and 3 correspond to values of calculated for heights h1, h2, and ha respectively.

The resultant voltage acting around the loop to produce night effect errors is er-ez. A vector diagram is given in Fig. 7 showing the relation.- ship between e1, e2, and (e1-e2). In this there has been some exaggeration of phase angles vfor the sake of clarity. To determine the magnitude and phase of (e1-e2) one can Adetermine the projections of e1 and e2 on the vector e1.

Sum of projection Since (e1-(32) is very small when hi/i and hz/l are small and (h1-h2) /k is even smaller, Equations 6 and 7 lead to The angle a by which er leads En in Fig. 7 is then Equations 8-11 show the magnitude and phase .of the voltage that gives rise to night effect error in an*v ordinary loop.

At the same time, the horizontally polarized downcoming wave that causes night eiect also induces in the auxiliary horizontal antenna a voltage e3 given by the equations A comparison of the voltages e3 and (e1.-e2)==er Vshows that both are proportional vto the magni- Examination of this expression shows that when h3=h1+h2, that the voltage `es induced inthe horizontal antenna has the same phase as the voltage (e1-e2) thaty produces night eiiect.Y

It is therefore possible that by combining output Voltage derived from an auxiliary horizontal antenna having a height ha=h1+h2, with the output voltage developed by the `loop antenna in the proper relative magnitudes and opposite phase, to. neutralize night effect present. However, even with haiULi-l-hz) the phase angle Iy given by Equation 14 is very small under ordinary conditions. Thus placing the auxiliary antenna midway between the upper and lower loop members (i. e. h3:

commonly introduces a phase angle of only a few degrees. When vii), itsr value is proportional to frequency, and if taken into account in the balancing operation between e3 and (e1-e2) by the use of a suitable phase shifting network, perfect balance is still possible. Even without such phase adjustment the balance `will be rather good. Furtherrnore,v the .adjustment giving neutralization is independent of the angle of incidence 6, and of the wave length A of the arriving wave. It will also be observed that the compensation thus effected is independent of the loop orientation, since changes in loop position affect al1 horizontal members proportionately.v

.The circuit of Fig.y 8 is van illustration Vof an embodiment of my invention. A .tunable auxiliary antenna l is connected to the control grid of an Aamplifier tube 2 whose plate is tied to the plate of ampliiier tube 3. The loop antenna 4 is connected to the control grid of amplifier tube @and the currents ,from both amplifier tubes appear in the common load 5, thereby causing addition of the voltages or currents of the auxiliary antenna and the loop antenna. The load circuit 5 is coupled to the following stages of the direction iinding receiver. The auxiliary antenna may be coupled directly to the loop antenna, but use of the amplier tubes 2 and 3 allows ready adjustment of the relative currents. This embodiment is shown by way of example and other means for connecting the two antennas may be employed by those skilled in the art without departing from the scope of my invention,

In the analysis that has been made the effect of mutual impedances between the loop and the auxiliary antenna has been neglected. While this is a modifying influence, it does not alter the resultsV obtainable, as Yby proper circuit Vdesign one can either compensate for the effect of the mutual impedance or can make its effects negligible.

While I have described particular embodiments of my invention for purposes of illustration, it will be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

What is claimed is:

1. An antenna system comprising a rotatable loop antenna having upper and lower horizontal members, said members being at predetermined heights above ground, a second horizontal antenna rotatable therewith and mounted thereon at a height in the order of the sum of the heights of said horizontal members and substantially in the plane of said loop antenna, and means for interconnecting said loop antenna and said second antenna in phase opposition.

2. A radio direction finding system having a receiving antenna system according to claim 1, wherein said interconnecting means comprises a'rrst amplifier stage, means for Vcoupling said second antenna to said stage, a second amplifier stage, means for coupling seaid loop antenna to said second stage in phase opposition with respect to the coupling of said second antenna, and a load circuit common to the outputs of both stages.

FREDERICK E. TERMAN. 

